When oxidizing organic saturated aliphatic aldehydes containing 5 to 9 carbon atoms to the corresponding monocarboxylic acids, an important objective is to obtain sufficiently high yields and product efficiencies at high conversion levels in a single pass, to avoid the necessity to recycle significant amounts of unreacted starting materials. Catalysts comprising copper and manganese facilitate this objective, since they result in the production of larger amounts of acid per pass than do manganese catalysts alone. However, a disadvantage which may result from the use of copper-manganese catalysts in aldehyde oxidation processes, particularly ones in which the reaction product must be distilled to recover the desired product, is the plating out of copper in the distillation apparatus. Plating out can lead to undesirable mechanical problems including erosion of reboilers and pump impellers, and rapid pump seal failures.
Copending U.S. application Ser. No. 345,890 filed Feb. 4, 1982, assigned to Celanese Corporation claims a process of the type described utilizing a copper-manganese catalyst. This process provides relatively high carbon efficiencies of aldehyde to acid at high aldehyde conversions. A single stage or two stage liquid phase reactor system generally gives sufficiently high aldehyde conversions so that recycle of unreacted aldehyde is, in most cases, unnecessary. However, when the reaction mixture is distilled to recover the acid, copper tends to precipitate and plate out on the distillation apparatus unless something is done to prevent it.
One means of overcoming this problem is to add oxalic acid to precipitate copper and manganese from the reaction mixture as their oxalates, prior to the distillation step. This process is described in U.S. Pat. No. 4,289,708, issued Sept. 15, 1981 to Scott et al and assigned to Celanese Corporation. Copper and manganese can also be separated from the reaction mixture by precipitating them, again as their oxalates, by adding an aqueous oxalic acid solution. In this case, the manganese and copper oxalates precipitate into the aqueous phase, which can be readily separated from the organic acid product by decantation. The acid can then be further purified by distillation. However, aqueous oxalic acid cannot be used satisfactorily to treat mixtures containing valeric acid due to this acid's high solubility in water. This process is described in U.S. Pat. No. 4,246,185, issued Jan. 20, 1980 to Wood, Jr. and assigned to Celanese Corporation.
It is the purpose of this invention to provide another technique for removing manganese and copper from C.sub.6 -C.sub.9 saturated aliphatic monocarboxylic acids and avoid the use of oxalic acid and the oxalate precipitates to remove the catalyst metal ions from the acid products.